I bet fuel pump - & while youre doing that, clean out fuel system & replace soft hoses as the can collapse internally -  I bet it'll drive all day at very low speeds. Had the same prob w several cars. Lost the engine on about half of them

You must have had lots more success with mechanics than I have.
do i like buying parts? not really
dizzy- electronic-upgrade
coil2- upgrade
fuel pump- $48- fresh start 
fuel filter $1.50
fuel lines
carb rebuild kit
those are the parts i've bought.  don't judge me too harshly.  I don't feel too bad about the parts I've bought.  upgrades i'd like to have done anyway. 

now.  how about those springs?

Do you enjoy buying parts? It seems to me that rather than guessing about what is wrong with your car you should now take it to a proffesional to have it diagnosed correctly? you can throw parts at this car all day long but a Pro maybe able to tell you on one test drive what your problem may be or at least eliminate more than a few causes! and as I've said before diagnosis via the forums is not always possible. your symtoms seem to be regular and not intermittent therefore straight forward to find. Not trying to say you wont fix it in the end but how much money and time before someone stumbles across the cause..................................

just for fun I installed a facet fuel pump.  not expecting it to fix it, but ill test it this week.  
my next thought is the car may have say for quite a while.  maybe some the the valve springs sat in a compressed position for too long. thinking it could be weak valve springs and getting some float.  
i double checked the valve clearances today. all seemed good.

it actually feels like a rev limiter.  it feels very similar to my cbr600rr motorcycle when i hit the limiter.  and like the cbr it'll run like that for a while.

suggestions on what set of springs to buy off the site?

Since you have a fuel filter in place, you can remove the tank screen. Drain and remove tank. Insert stiff wire or small rod into outlet pipe and push screen off inner end. Shake from tank and discard. Reinstall tank.

to reconfirm a few things-
-tank outlet screen NOT yet confirmed clear.
-no pinch in fuel lines
-choke connected correctly and tested- problem remains
-bought and tried new coil-  problem remains

MLM for short.  That's the name of the car BTW.  Read the book Pandora's Star.  amazing.  I'm Steve.
I've thought about the valve float possibility. That's next for me to check.
thanks for the input. 
I ran a fresh wire from the fuse block to the coil.  no difference.  
Next week i'll try disconnecting the tach wire and running a direct wire from a hot to the coil (temporarily). 
the battery shows 12.4v, but through the ignition shows about 11v. (key on car car not running)

To recap...
O/P = original poster = Morninglightmountain (I'm only gonna type that once!)

1. Fuel & Carburetion:
 - tank vent tested OK. 
 - tank outlet screen suggested by D. Lawson but not confirmed clear.
 - remainder of fuel line cleaned and blown free, though no confirmation there is no pinch.
 - electric fuel pump confirmed OK for flow
 - rubber hoses replaced.
 - carb inspected, cleaned, including inlet jet screen, overhauled and reinstalled with connections confirmed correct.
 - choke test at problem rpm did not improve, though O/P said choke cable was not working properly - was this corrected and retested?
 - K+N type filter deemed not a significant issue, though needle may not be correct - choke test should overcome this as an issue.
 - carb float set
2. Ignition:
 - Flamethrower coil. O/P was going to buy a different coil and "test tomorrow/this weekend", but has not reported results of different coil. Also was replacing coil supply wire from fuse box.
 - Pertronix electronic ignition. O/P asked for confirmation of wire connections but has not had response or reported a change.
 - O/P confirms timing OK and advance functioning appropriately.
 
3. Other possibilities:
 - O/P has not reviewed what else is connected to the coil, e.g. tach. Should disconnect to verify spark is not being compromised by a fault somewhere else.
 - remote possibility: weak valve spring allowing valve float at higher rpm under load. symptom of suddenly  "running on 3 cylinders" might suggest that.
 - really remote possibility: EMF interference at 38th street exit causing electronic ignition to malfunction. See the International Mini Film Festival 2016 winner. https://vimeo.com/187431558

Taking things in the logical order now we know the filters are clear and the float is set about right - What height is the jet set to ?

I had some time to think about since yesterday. This is an Authi and was imported from Spain. All Metro engine swap was done there. It came with a Metro cast as one intake/exhaust and HIF 44. The 850 exhaust was attached to the metro down pipe. At least they greased carb. and firewall where they made contact all the time. As dropped off to me it would not run over 40 mph. Starting at the cyl. head I changed the intake to a MSC single carb manifold. Added a LCB and complete new exhaust system. The HIF 44 would not clear bulkhead with this intake either. Changed to HS 6 with K&N cone with 1/4 spacer. With everything clearing and the engine revving nicely without a load out for test drive. Before I made carb or timing adjustments I ran a compression test. 175/ 180/180/180. 

As I mentioned the fuel line under the car has been mashed in several places. It may have gone Ro Ro and bottomed on a ramp someplace. The exhaust (old 850) was broken up under car as well. A new exhaust system was reason for shop visit. 

I'm thinking this is a fuel issue, when you hit 60 + - it dies and if you press gas pedal harder you can hear the K&N sucking but nothing happens till you lift and let it catch it's breath. 

This engine has the electronic dizzy with the amplifier on the side. I have heard hooked up backwards affects the advance curve. 

Removing a section of the fuel line mashed almost flat in a couple of spots did not change anything. 

I ran a 998 with a mech. pump for years with no issue. No place for mech. pump on my Cooper S engine so went to electric pump. I have held my mini at 95 mph for 20 min without fuel issue. 

Plan is to replace complete fuel line and add a electric pump and by pass mech. pump. Next visit she gets a set of S brakes and complete adjustable suspension. Thinking about it 50 mph may be a good speed for a drum brake car. Steve (CTR)

sounds similar to my issue.  please me top to date on your progress.

Took it for test run today. Runs fine at 50 mph move up to 60 and it dies let up and it will come back. Cut two of the kinks out and ran a new section, still the same. Changed needles for richer same deal. Metro power unit, HS 6 mech. pump. Steve (CTR)

I had a similar issue with my HIF 44 for quite a while.  Bill at Mincomp found a tiny piece of plastic in the lower plastic part of the jet that was apparently scrap from the manufacturing process.   It would bounce around in there and clog up everything during high speed. When he took the carb apart he heard something rattling in the jet. He put in a new jet and it ran perfectly.   

I've replaced all rubber lines and blew compressed air thru the solids.  they looked good also.
I'm going to try a facet pump (mostly to put my mind at ease) then i'm getting a vtec if that doesn't do it (joke).

I double checked the timing this morning and took it for a spin.  I get on the highway and the second I hit the 38th St. exit it starts missing.  Every.... Single... Time...  I could set my watch to it. 

If the pump doesn't do it i'm going to try to borrow a different carb.  Mine looks good to me but I don't have a lot of experience.  I could be missing something. 

Thanks for all the input fellas!

I have not read this whole thread, but working under a customer car this week I noticed his fuel line appeared crushed or kinked around the front sub frame. Current shop work was rebuild rod change shifter and install new exhaust. Next visit will include suspension rebuild and disc brakes. May take a look at replacing fuel line then. Steve (CTR)

it's a new pertronix dizzy w' flamethrower coil.
lot's of power.
I've checked the timing with advance disconnected. total advance right about 30 at 4krpm.
i got a new coil and have run a new wire from the fuse block to the dizzy. the old one looked iffy but did have continuity. 
Hopefully try it tomorrow morning.
Thanks for all the advice guys.  I'll let you know if i get this thing sorted. 

Correct,  and the total timing needs to be set first before the initial timing.    Initial timing is just a referrence point.

https://youtu.be/kgv0wHE895w 

Have you checked total timing? ie above 3,000 RPM with Vac. advance disconnected? if the distributor weights are seized or sticking this could be your problem at speed you could be having high combustion chamber Temp's. overheating the plugs and causing your missfire/lack of power?

i laid my straight edge across the carb and adjusted the float with about 1mm clearance at the scoop. 

I have a good dial back gun & double checked it with a friends in case it was off.